1. Field of the Invention
This invention relates to a head driving device of an ink-jet printer having an ink-jet head which utilizes electrostrictive elements for causing variations in pressure in ink chambers by the distorting operation thereof.
2. Description of the Related Art
A line thermal printer is known as a line printer having a larger number of recording elements arranged in a line. As shown in FIG. 11, the line thermal printer has a system bus 1 which is connected to a CPU (Central Processing Unit) 2 constructing the main body of a control section, a ROM (Read Only Memory) 3 storing program data and the like, an interface (I/F) 4 for effecting the transmission/reception control with respect to an external host computer (not shown) and receiving a printing command and print data from the host computer, an image RAM (Random Access Memory) 5 for storing image data created by developing received print data into a bit map form, and an ASIC 7 for supplying an energization signal to a driver 6 which controls energization of heating elements of the line thermal head.
The driver 6 is constructed by cascade-connecting a large number of driver ICs 6a each including a shift register 8, latch circuit 9, AND gate circuit 10, and switching circuit 11 as shown in FIG. 12. That is, the driver 6 is constructed by connecting a large number of driver ICs 6s such that the data output terminal DO of a preceding stage driver IC 6a is connected to the data input terminal DI of a next stage driver IC 6a.
The thermal line printer transfers image data read out from the image RAM 5 via the ASIC 7 to the data input terminal DI of the driver 6, and stores them in the shift register 8 which sequentially shifts the image data in response to a clock CK. When a series of image data is stored into the shift register 8, the data stored therein are latched by the latch circuit 9 in response to a latch signal LT. Then, outputs of the logical AND of the outputs of the latch circuit 9 and an energization signal FIRE are supplied from the AND circuit 10 to respective switching elements of the switching circuit 11, thereby selectively turning ON the switching elements to supply print outputs. The heat-generating elements perform a printing of dots by selectively generating heat according to the print outputs. That is, as shown in FIG. 13, each of heat-generating elements 12 is connected in series with a switching element 13 such as an FET between +VCC and ground terminals. Energization of the heat-generating element 12 is controlled by setting the switching element 13 in the ON state for a preset period of time by an energization signal F from a corresponding one of the AND gates of the AND circuit 10. FIG. 14 shows the timings of the latch data, energization signal FIRE, and print output used in the above operation. FIG. 15 shows the timings of the image data stored into the shift register 8 via the data input terminal DI, clock CK, latch signal LT, latch data, energization signal FIRE and print output. However, the head driving device of the above line thermal printer simply controls energization or de-energization of the heat-generating elements.
As the head driving device of an ink-jet printer having the ink-jet head using a piezoelectric or electrostrictive elements, a head driving device of a serial ink-jet printer is known. For example, Jpn. Pat. Appln. KOKAI Publication No. 6-286136 discloses an ink-jet head is constituted by a column of ink chambers having electrodes formed therein and separated by partition walls of piezoelectric elements (the number of ink chambers corresponds to a number of vertical dots required for one-line printing). Each electrode of the head is connected to a driving circuit formed of switching elements respectively serving as charging and discharging circuits. Each of the piezoelectric element is sequentially distorted by sequentially changing a voltage applied to a corresponding one of the electrodes of the head to different voltage levels, for example, +V.fwdarw.0.fwdarw.-V/2, according to a sequence of ON and OFF states of the switching elements to apply pressure to the ink chamber and eject ink in the ink chamber so as to effect the dot printing operation. When the partition wall of the piezoelectric element is distorted to apply pressure to the ink chamber, the volume of an ink chamber adjacent to the above ink chamber increases to set a negative pressure state so that the control operation for simultaneously applying pressure to both of the adjacent ink chambers cannot be effected in the case of an ink-jet head using the piezoelectric element as the partition wall, and therefore, the dot printing operation for one column is effected by a so-called two-cycle driving method in which the process for operating the alternate ink chambers is effected twice, for example.
When the gradation printing is performed for each printing dot by use of a printer using the ink-jet head for ejecting ink by utilizing the electrostriction of the piezoelectric element, signals for controlling voltages applied to the piezoelectric elements and the application timings thereof, that is, a sequence of the ON and OFF operations of a plurality of switching elements of the driving circuit and the time lengths of the ON and OFF states are issued from control circuits provided for the respective dots and control signals which are different for respective dots and correspond in number to the dots are prepared and used for changing the degrees of electrostriction of the piezoelectric elements and the timings of electrostriction thereof.
However, since the head driving device of the serial ink-jet printer using the piezoelectric elements effects the operation for distorting the piezoelectric elements of the ink-jet head for each column and repeatedly effecting the distorting operations for one line to attain the one-line printing, a relatively long time is required for the printing, and if an attempt is made to attain a certain printing speed, restrictions are imposed on the permissible time for the operation of the piezoelectric elements of one column and the electrostriction degrees of the piezoelectric elements and the electrostriction timings thereof cannot be sufficiently changed, thereby causing a problem that satisfactory gradation printing cannot be attained. On the other hand, if an attempt is made to set the permissible time for the operation of the piezoelectric elements of one column sufficiently long so as to sufficiently change the electrostriction degrees of the piezoelectric elements and the electrostriction timings thereof, there occurs a problem that the printing speed is lowered.
Further, in order to perform the gradation printing for each printing dot, it is necessary to simultaneously prepare voltage levels and timings which are different for respective dots in one column and control them for respective dots of one line while changing them at the column frequency, thus causing a problem that the control operation becomes complicated.